Monoclonal antibodies directed against neurofilament (NF-M) proteins in Xenopus, have been injected into individual blastomeres of Xenopus embryos at the two-cell stage. These antibodies remain restricted in the intracellular space so that only cells derived from the injected blastomere contain the antibody (i.e, about half the cells in the developing embryo contain antibody). The results are that the axons derived from neurons containing these antibodies show a variety of dramatic deficiencies in axonal outgrowth and elongation, representing the first demonstration in vivo that perturbation of a neurofilament protein has structural consequences. Studies on rat central and peripheral nervous system neurons in vivo and in vitro have shown that different post-translational modifications (i.e., phosphorylation of specific sites) of neurofilaments are characteristic of specific nerve pathways and neuronal types, and not necessarily correlated with axonal caliber or their central or peripheral origins. Dissociated sensory ganglion cells from postnatal rats survive in defined media in the absence of NGF, but are deficient in general growth, neuropeptide and neurofilament expression and neuritic outgrowth.